In image forming apparatuses such as electrophotographic copiers, electrostatic recording devises, or magnetic recording devices, two-component type development devices using two-component developer are widely used for developing electrostatic latent images formed on latent image bearers.
Such two-component development devices typically include a developer bearer rotatable relative to a casing of the development device, a stationary magnetic field generator provided inside the developer bearer, and a developer regulator disposed across a gap (regulation gap) from the surface of the developer bearer, upstream in the direction of rotation of the developer bearer from a development range facing a latent image bearer. The magnetic field generator has multiple magnetic poles and may be constructed of multiple magnets. The magnetic field generator includes an attraction pole or pump-up pole for generating a magnetic force to attract the developer (i.e., developer particles) to the surface of the developer bearer (hereinafter “attraction magnetic force”) and a development pole for generating a magnetic force to cause the developer to stand on end on the developer bearer in the development range.
With the magnetic force generated by the magnetic field generator, the developer is carried on the surface of the developer bearer and transported to the development range. In the development range, the developer standing on end on the developer bearer forms a magnetic brush, which slidingly contacts the surface of the latent image bearer. Then, toner in the developer adheres to the electrostatic latent image formed on the latent image bearer, thus developing it into a toner image (development process).
For example, JP-2008-256813-A proposes a two-component development device in which a developer supply compartment and a developer collection compartment are formed by the casing and interior wall therein, and conveyance screws (i.e., developer supply screw and developer collecting screw) are provided therein. The developer supply compartment is positioned adjacent to the developer bearer, and a side wall of the developer supply compartment or a partition divides, at least partially, the developer supply compartment from the portion where the developer bearer is provided. The developer supply screw supplies the developer from the developer supply compartment to the developer bearer while transporting the developer in the axial direction of the developer bearer. The developer in the developer supply compartment overstrides the side wall and is carried on the surface of the developer bearer due to the attraction magnetic force.
As the developer bearer rotates, the developer reaches the regulation gap, which is a gap between the surface of the developer bearer and the developer regulator. Only the developer adjacent to the surface of the developer bearer can pass through the regulation gap, and the developer positioned away from the surface of the developer bearer is blocked by the developer regulator. Thus, with the regulation gap, the amount of developer transported to the development range can be adjusted, and the developer removed by the developer regulator from the developer bearer is returned to the supply compartment and is again supplied to the developer bearer. Thus, the developer is circulated inside the development device.
The amount of developer transported to the regulation gap, however, fluctuates when the properties of the developer, such as fluidity, change due to the degradation of the developer over time or changes in the environment. In this case, the development ability becomes unstable.
In view of the foregoing, several approaches have been tried. For example, the magnetic field generator may be configured to have another magnetic pole for generating a magnetic force to cause the developer to stand on end on the developer bearer (hereinafter “regulation magnetic force”) when the developer passes through the regulation gap to alleviate the fluctuation in the amount of developer supplied to the development range.
Although this approach is effective to a certain extent, the regulation magnetic force can also act on the developer blocked by the developer regulator, retaining such developer (hereinafter “retained developer”) in a portion downstream from the developer regulator in the direction of rotation of the developer bearer (hereinafter “retaining portion”). In the retaining portion, the retained developer is circulated in the direction opposite the direction of rotation of the developer bearer. While thus retained by the regulation magnetic force and circulating in the retaining portion, the retained developer is further electrically changed by sliding contact. Accordingly, the amount of charge of the toner in the retained developer is higher than that of the other developer circulated in the development device, and thus the development ability, that is, the amount per unit area of toner adhering to the electrostatic latent image during the development process, is different therebetween.
Although unevenness in image density can be limited as long as such developers having different levels of development ability are mixed well, the unevenness in image density is visible if they are mixed insufficiently, degrading the image quality. In conventional development devices, it may be difficult to sufficiently mix developers having different levels of development ability. Consequently, unevenness in image density can occur, and accordingly the image quality can be degraded.